G.R. Massoumi

An alternative technique for oxygen surface coverage measurements

Abstract Absolute cross sections and angular distributions have been measured with a precision of ∼ 4% for the 16O(3He, α)15O and 16O(3He, p)18F nuclear reactions at an energy corresponding to a weak resonance, E( 3 He) ≈ 2.4 MeV . The total cross section summed over p0−p6 and α approaches 4 mb/sr at backward detection angles. These reactions offer significant advantages over the 16O(d,p1)17O reaction for oxygen surface coverage measurements, particularly on low-atomic-number substrates.

IMPROVED STOICHIOMETRY MEASUREMENTS USING 4HE ELASTIC BACKSCATTERING : EXPERIMENT AND SIMULATION

Abstract We have measured absolute spectra for MeV energy 4He ions backscattered from amorphous silicon, SiO2 and Si3N4 targets. The simulated backscattering spectra from a-Si are only brought into acceptable agreement with experiment when we adopt 4He stopping cross section values recently reported by Konac et al. [Nucl. Instr. and Meth. B 136–138 (1998) 159]. When these same values are used to simulate RBS spectra from SiO2 and Si3N4 targets, good agreement is found for ions incident at 2 MeV but there is ∼8% disagreement for 1 MeV ions. The implications for stoichiometry determinations based on RBS are discussed.

Revisiting the 12C(d, p)13C reaction cross section using condensed gas targets

Abstract We have measured the differential cross section for the 12C(d, p)13C nuclear reaction at a laboratory scattering angle of 150° for incident deuteron energies near 972 keV. The measurements have been calibrated by comparison with the well known cross section for the 16O(d, P1)17O reaction at the same energy (972 keV) and angle, corresponding to the peak of a broad resonance in the latter reaction cross section. The technique utilized condensed targets of gases containing both carbon and oxygen atoms, viz. CO, CO2, and CH3OH. The three values thus obtained for the 12C(d, p)13C laboratory cross section at 972 keV are in excellent agreement (± 2%). A mean value of 27.8 ± 1.1 mb sr−1 is recommended. We conclude that, for CO adsorbed on the Pt(100) and Pt(111) surfaces at 185 K, the reported saturation coverages were stoichiometric.

Anomalous pulse heights in silicon detectors

Abstract Accurate measurements of the relative pulse heights produced by alpha particles from radioactive sources (148Gd and 241Am) in a silicon surface-barrier detector show that the detector response is not linear in the deposited energy. A semi-empirical model, wherein the average energy to create an electron-hole pair in Si is dependent on the energy loss rate of the ionizing particle, is in fair agreement with the experimental data.

The determination of absolute oxygen coverage by nuclear reaction analysis

Abstract The saturation of a Ni(100) surface with oxygen is demonstrated by NRA for both 16 O and 18 O. A value of 2.5 monolayers is found. The same saturation value is obtained for a surface first converted to the chemisorbed c(2 × 2) phase in 18 O, then taken to saturation in 16 O. This simple demonstration suggests that surface reaction mechanisms may be studied through isotopic labelling in conjuction with NRA methods.

Absolute determination of saturated ethylene coverage on Pt(111)

Abstract We have measured directly the absolute surface coverage of ethylene adsorbed to saturation on a clean Pt(111) surface at 100 K. The fully deuterated compound (C 2 D 4 ) was substituted for C 2 H 4 , enabling quantitative measurements to be made of both C and D areal densities via two independent nuclear reactions. For the C atom coverage, we have measured the 12 C(d,p) 13 C yield for incident deuterons of 972 keV energy. For the D atom coverage, we have measured the D( 3 He, p) 4 He yield for incident 3 He ions of 630 keV energy. Measurements were made in conjunction with LEED (at 300 K), Auger and thermal desorption spectroscopies. For ethylene adsorption at 100 K, the saturation carbon and deuterium coverages are consistent with C 2 D 4 stoichiometry and the observed coverage was 0.23 ± 0.02 monolayers. The coverage for adsorption at 300 K, assuming C 2 D 3 stoichiometry, was measured to be 0.21 ± 0.02 monolayers.

On the calibration of low-energy ion accelerators

Abstract The pulse height measured in a silicon detector for MeV 4He ions backscattered from a monoisotopic target, e.g. Au, has been compared to that from a radioactive 241Am source. Using previous results for the so-called “pulse height defect”, including the effects of nonlinear detector response, we have determined the incident beam energy at ∼ 2.4 MeV with a precision of ∼ 3 keV. The data are in good agreement with a calibration performed using the 27Al(p, γ)28Si nuclear reaction resonance technique.

Characterization of silicon oxynitride films using ion beam analysis techniques

Abstract Heavy-ion elastic recoil detection analysis (HIERDA) is the ideal technique for quantitative analysis of silicon oxynitride films on silicon because of its unique ability to measure simultaneously all elements of interest (i.e., H, C, N, O and Si), thereby permitting key parameters such as the O/N-ratio to be determined in a single measurement. However, high-energy accelerators suitable for such HIERDA measurements are becoming much less readily available. Hence, the present paper investigates and calibrates an alternative IBA technique for simultaneous O, N and C analysis – namely, the use of (d,p) and (d,α) nuclear reactions. Under optimum analysis conditions (850 keV deuterons and 150° detector angle), the Si background level sets a lower detection limit of ∼1×10 16 nitrogen atoms/cm 2 and ∼3×10 15 oxygen atoms/cm 2 . H analysis is carried out separately, using low-energy ERDA and a 2 MeV 4 He beam. Absolute cross-sections have been obtained for each of the (d,p) and (d,α) groups. Comparison with data in the recent Handbook of Modern Ion Beam Materials Analysis shows reasonable agreement (10–15%) for the (d,p) reactions on oxygen and carbon. However, in the case of nitrogen, the measured cross-section values are ∼70% larger than the Handbook data. Several silicon oxynitride samples have been analyzed, first at UWO using 850 keV deuterons, and subsequently at ANU using HIERDA and a 200 MeV Au beam. The resulting O/N-ratios agree to within 10%. The relative importance of radiation damage effects is briefly discussed.

Positron emission yields for encapsulated 22Na sources

Abstract We have measured the total γ-activity and the emitted positron yields from two commercial 22Na sources (∼ 5 mCi and ∼ 70 mCi). For the weaker source, we find good agreement between the number of positrons expected (based on γ-yield measurements) and those observed, after estimating the effect of positron absorption in the Ti window of the source capsule. The yield of positrons from the stronger source is a factor of two lower than expected. Reasons for the discrepancy are discussed.

Absolute coverage measurements on sulphur-passivated GaAs(100)

Abstract Particle-induced X-ray emission measurements combined with Rutherford backscattering spectrometry (including channeling) have been used to measure directly the total sulphur coverage for the S-passivated GaAs(100) surface. The ion-induced X-ray measurements show that 1.1 ML (1 ML = 6.26 × 1014 cm−2) of S is found on GaAs(100) after passivation with (NH4)2S, while only 0.55 ML of S is present on the H2Sx-treated GaAs(100) surface. A clearer picture of the S-covered GaAs(100) surface emerges from a consideration of these data in conjunction with current models encompassing surface dimers.